The present invention is directed to plastic bottles used to contain foods and beverages that include vacuum responsive panels designed to compensate for temperature induced changes in internal conditions subsequent to a filling and capping operation that occurs with the contents of the bottles at an elevated temperature.
Lightweight, thin-walled containers made of thermoplastic materials such as polyester resin are well known in the container industry. For example, polyethylene terephthalate (PET) has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics beverages and so on. PET can be molded, by orientation-blowing, into transparent thin-walled containers having a high stiffness, impact strength and improved hygienic qualities with a high molding accuracy. Strong, transparent and substantially heat resistant containers may be produced by the biaxial-orientation blow-molding process in which a parison is oriented both laterally and longitudinally in a temperature range suitable for such orientation. Heat-set PET containers are particularly heat resistant. Biaxially-oriented blow-molded containers have greater stiffness and strength as well as improved gas barrier properties and transparency.
When a thermoplastic container is filled with a hot liquid (such as a liquid sterilized or Pasteurized at a high temperature) and sealed, i.e. hot-filled, subsequent thermal contraction of the liquid upon cooling results in partial evacuation of the container which tends to deform the container walls. Such deformation typically concentrates at the mechanically weaker portions of the container, which can result in an irregular and commercially unacceptable appearance. Further, if the deformation occurs in an area where the label is attached to the container, the appearance of the label may be adversely affected as a result of container deformation. By increasing the wall thickness of the container it is possible, to some extent, to strengthen the container walls and thus decrease the effects of vacuum deformation. However, increasing the wall thickness results in a substantial increase in the amount of raw materials required to produce the container and a substantial decrease in production speed. The resultant increased costs are not acceptable to the container industry.
Prior art approaches have included the use of collapse panels, i.e., indented surface areas which provide for controlled, quantified collapse to overcome thermal deformation. The collapse panels are typically spaced around the perimeter of the container by intervening lands. However, problems have developed in containers designed with collapse panels. While collapse panels accommodate a great degree of controlled deformation, as the vacuum inside the containers increases, more and more collapse is required from the collapse panels without permitting collapse of the intervening lands. A variety of structures have been adopted to focus the vacuum collapse solely within the panels. For example, U.S. Pat. No. 4,805,788 discloses a bottle wherein the walls contain collapse panels, and the collapse panels contain ribs to accommodate a high degree of evacuation of the container without deleterious changes in the container's rigidity or appearance. The ribs extended longitudinally at the sides of the collapse panels so as to isolate the movement in the collapse panels from the intervening lands.
In U.S. Pat. No. 4,863,046, longitudinal ribs are included in the center of each of the intervening lands. Additionally, lateral ribs are included in the panels to reinforce the panels against pressure or vacuum deformation. The lateral ribs in the panels are also disclosed in U.S. Pat. No. 5,005,716 and U.S. Pat. No. 5,178,290. In U.S. Pat. No. 5,092,475, the longitudinal ribs included in the center of each of the intervening lands are extended vertically beyond the vertical extent of the collapse panels. Further, the collapse panels, located in the portion of the bottle designed to accept an overlying label, include a radially inwardly offset peripheral portion from which a central boss portion projects radially outward to an outer panel, which can be located at about the same radial position as the intervening lands. The boss outer panel, which is generally rectangular with rounded corners, acts to support the overlying label and can be reinforced by a radially inwardly extending, vertical rib extending over a substantial portion of the outer panel. In U.S. Pat. No. 5,178,289, vertical stiffening ribs are disclosed in both the intervening lands and in the center of the outwardly projecting boss portions of the flex panels. Horizontal stiffening ribs are disclosed in the outwardly projecting boss portions of the flex panels in U.S. Pat. No. 5,762,221.
In U.S. Pat. No. 5,337,909 the problem of deformation of the container sidewall during vacuum compensation following hot-fill is addressed by providing circumferentially extending inwardly directed reinforcement ribs located in the immediate vicinity of, or even intersecting, the upper and lower margins of the vacuum compensation panels. A similar approach was used in U.S. Pat. No. 5,704,503 with the added element of vertically oriented longitudinal ribs in the posts between the vacuum compensation panels. Multiple discontinuous horizontal reinforcement ribs located at about the same location was disclosed in U.S. Pat. No. 6,036,037. The amount of total panel deflection can be computed based on the volume and temperature changes that are expected to occur in the container, and thus the size of the panels can be specifically scaled to accommodate the anticipated vacuum. Despite these and other attempts at structural solutions for the sidewall deformation problem, the problem persists to varying degrees in a wide variety of hot-fill containers that are commercially sold.